1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to a nonaqueous electrolyte for alkali metal electrochemical cells. A preferred form of the present invention relates to an electrolyte for rechargeable alkali metal electrochemical cells such as rechargeable lithium ion cells.
2. Prior Art
The choice of electrolyte solvent is one of the most important factors in determining cell discharge performance characteristics. For rechargeable lithium ion cells, sometimes referred to as "rocking chair" cells, nonaqueous organic electrolytes containing carbonate solvents, for example propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) are generally used because of their high oxidative stability toward cathode active materials and their high kinetic stability toward the alkali metal anode. The latter characteristic is believed to be caused by anode surface passivation.
Recently, it has been demonstrated that the utilization of an unsymmetric linear carbonate, such as ethyl methyl carbonate (EMC), as a co-solvent in a nonaqueous electrolyte in many cases provides improved cell discharge characteristics including increased energy density, increased discharge capacity, longer cycle life and higher safety performance in comparison to the conventional solvents discussed above. This has been reported in the literature including published European Patent Application No. 0622862A1; U.S. Pat. No. 5,484,669 to Okuno et al. and U.S. Pat. No. 5,521,027 Okuno et al.; and Japanese Laid-Open Patent Publication Nos. 2-148665, 6-290809, 5-299118, 7-296848, 7-288140. Although unsymmetric linear carbonates have been demonstrated to be acceptable co-solvents for nonaqueous electrolytes activating rechargeable alkali metal ion cells, the availability and the relatively high price of these solvents has limited their wide spread application.
A well known method for synthesizing an unsymmetric carbonate is the esterification of alkyl chloroformate with alcohol under base (pyridine or amine) catalysis, as shown in equation 1. This synthesis is reported in the literature including: Edmund P. Woo and Frank C. W. Cheng, J. Org. Chem. 1986, 51, 3704-3706; and Ichiro Minami and Jiro Tsuji, Tetrahedron 1987, 43, 3903-3915. ##STR1##
The esterification reaction of equation 1 requires very reactive and highly toxic starting materials that necessitate strictly controlled reaction conditions. Consequently, that esterification reaction is not a desirable synthetic route for large scale production of unsymmetric carbonates.
Another method for unsymmetric linear carbonate synthesis is disclosed in Japanese Laid-Open Patent Publication No. 6-166660, which reports an ester exchange reaction of a dialkyl carbonate with alcohol under a weak base (metal ion carbonate salt) catalysis, as shown in equation 2. ##STR2##
The reaction in equation 2 proceeds under fairly mild conditions. The starting materials are less toxic than those disclosed in equation 1 and are relatively easy to handle. However, the disadvantage of an ester exchange reaction of a dialkyl carbonate with alcohol under base catalysis is that the synthesis products include a mixture of three dialkyl carbonates and two alcohols. For instance, when a 1:1 molar ratio of DMC and ethanol is used as the starting materials, the product mixture contains a 46:44:10 molar ratio of DMC:EMC:DEC and the relative ratio of methanol and ethanol, theoretically, is 64:34. The molar ratio of the desired EMC product is, thus, only about 22% of the resulting mixture. Since alcohol is an undesirable impurity in any electrolyte intended to activate an alkali metal cell and in particular lithium primary cells and lithium ion rechargeable cells, vigorous purification of the carbonate products to remove the undesired alcohols is needed. Sometimes, this goal is very difficult to achieve due to the similar boiling points of the reaction mixture components.
According to the present invention, the above discussed problems and difficulties of the prior art preparation techniques are overcome by synthesizing unsymmetric carbonates using a new methodology. Increased product yield and an alcohol free solvent are obtained according to the present invention. In addition, the reaction product mixture of the present invention can also be used directly as a battery electrolyte solvent or co-solvent without further purification.